In the electrocoating art an organic coating, i.e., a coating comprising a resinous organic binder and optionally containing other conventional coating components such as pigments, extenders, cross-linking agents, mildeweides, etc., is formed upon an electrically conductive workpiece by electrodeposition from a liquid (usually aqueous) dispersion. Following electrodeposition of the coating material onto the workpiece-electrode, the workpiece is removed from contact with the liquid electrocoating dispersion, rinsed to remove adhering portions of that liquid dispersion and, usually, subjected to a curing operation to crosslink the electrodeposited binder resin so as to convert the coating on the workpiece to an insoluble, hard, adherent, protective film.
The liquid dispersion contains an ionized form of the binder resin dispersed in the liquid medium together with the other components of the coating to be deposited and also contains a counter-ion soluble in the liquid dispersion medium. The ionized resin may be either truly dissolved or merely finely dispersed as an emulsion or a colloidal dispersion in the liquid dispersion medium. It is preferred that the counter-ion be truly dissolved in the liquid dispersion medium, as this will lead to the greatest stability of the dispersion of the ionized resin species. The electrically conductive workpiece to be electrocoated is connected through a suitable external circuit including a direct current emf source to a counter-electrode and both workpiece and counter-electrode are brought into contact with a body of the liquid dispersion. When the emf source in the external circuit is energized so as to charge the workpiece-electrode (relative to the counter-electrode) to a polarity opposite to the charge carried by the ionized binder resin in the liquid dispersion, then that ionized binder resin will electrophoretically migrate toward the workpiece-electrode and, if the voltage is great enough, will electrodeposit thereon by electrical neutralization in the vicinity of the workpiece-dispersion interface. It is found that by suitable selection of materials, the other optional and un-ionized components of the dispersion can be carried into the coating thus formed on the workpiece electrode.
In anodic electrocoating the workpiece-electrode is given a positive electrical charge relative to the counter-electrode and the binder resin contains groups ionizable to form anions. These are most often carboxylic acid groups and they are ionized by adding to the dispersion a base, such as an alkali or an amine, soluble in the dispersion medium. In cathodic electrocoating the resinous binder contains groups capable of ionizing to form cations and the actual degree of ionization of these groups in the dispersion is enhanced by the incorporation in the dispersion of material ionizable to form anions soluble in the liquid medium. Ordinarily the ionizable groups on the resinous binder are amine groups and the ionizing material is an acid which forms water soluble anions.
The emf source in the external circuit is used to charge the workpiece electrode as an anode or a cathode relative to the counter-electrode while both are in contact with a body of the liquid dispersion. The ionized resinous binder in the dispersion is electrophoretically attracted toward the workpiece-electrode and deposited thereon by electrical neutralization at the electrode-dispersion interface, while the counter-ions are simultaneously electrophoretically repelled therefrom, although a small proportion of the counter-ion or at least of counter-ion generating species is known to sometimes deposit on the workpiece electrode nonetheless. Other conventional coating components may be present in the liquid dispersion and, if suitably selected, will be co-deposited with the electrically neutralized form of the ionized resin upon the workpiece electrode to form the coating thereon, even though in the dispersion these other optional components are now known to bear charges. The mechanisms by which these other components are deposited as part of the coating are not fully understood, but in some cases such co-deposition may arise because the component is soluble the ionized binder resin which is itself present as a finely dispersed second phase rather than as a true solution in the aqueous medium. In other cases the ionized binder resin may attach by absorption to the other component, thereby conferring electrophoretic responsiveness upon an otherwise uncharged species.
A wide variety of resin types can be used as the binder component in electrocoating compositions for either anodic or cathodic deposition, so long as they contain a sufficient number of ionizable groups of the appropriate polarity, namely anionic for anodic deposition and cationic for cathodic deposition, to impart a useful degree of electrophoretic mobility.
The cure of organic coatings containing resinous binders having active hydrogens by thermally induced cross-linking reaction with aminoplast resins is well known. The active hydrogens in the binder resin to be cross-linked are typically provided by incorporating hydroxyl, carboxyl, or primary amide groups in that resin. In other respects the composition of the binder resin is almost unrestricted, insofar as curability is concerned, so long as a reasonable intimacy of admixture with the aminoplast resin can be achieved. Thus curing with aminoplast resins may be used in a wide variety of coating compositions wherein the binder resin may be selected from a wide range of compositions on the basis of availability, cost, application properties, and performance properties of the final cured coating. Typical resins for use in coating compositions to be cured with aminoplast resins include epoxies, polyesters, alkyds (especially maleinized alkyds), acid- or amide-functional acrylics, and many others.
Aminoplast resins useful for curing organic coatings by cross-linking the binder resins of such coatings to form insoluble, high molecular weight materials having a wide range of properties, depending on the type of resinous binder, the type of aminoplast resin, their relative amounts, the other components of the coating composition, the conditions under which cure is achieved, etc., are well known in the coatings art. Typical aminoplast resins for such purposes are derived from polyfunctional amines or amides by reaction with formaldehyde. Among the more important aminoplast resins used in the organic coatings art are reaction products of formaldehyde with urea or with aminotriazines, such as melamine or benzoguanamine, and low molecular weight polymers thereof, and ethers of any of these with lower alkanols, especially methanol where it is desired to enhance water solubility and butanol where it is desired to suppress water solubility.
While the conditions of temperature and time required for cure of any particular coating composition by reaction of an aminoplast resin component with a resin component having active hydrogens will vary somewhat depending upon the particular chemical structures and concentrations involved, it is widely recognized that incorporation of acids in such compositions will tend to reduce the temperature and/or the time required to achieve cure as compared to the same composition in the absence of such acids. In general, the stronger the acid, and the higher its concentration in the coating, the greater will be the reduction in the temperature and/or time required for cure.
Aminoplast resins have been used to cure coatings electrodeposited upon an anode from an aqueous dispersion of the aminoplast resin and an anionic form of a binder resin containing active hydrogens. Dispersion of the anionic resin in the aqueous medium is ordinarily stabilized by the presence of a water soluble cation derived from an added base, such as an amine or alkali. The anions on the resin in the aqueous dispersion are usually deprotonated carboxyl groups and it is thought that the carboxylic acid groups are regenerated therefrom by electrical neutralization at the anode during electrodeposition of the coating. These carboxylic acid groups are then perforce available to catalyze the curing reaction between the binder resin and the aminoplast resin co-deposited therewith upon the anode when that coating is subsequently baked. The cure reaction may, of course, involve reaction of the acid groups themselves or of other active hydrogen-bearing groups on the binder resin or both with the aminoplast resin.
An improvement in the anodic electrocoating art, wherein the aminoplast resin also has ionizable carboxylic acid groups, has been described by Coates et. al. in U.S. Pat. No. 3,519,627 granted July 7, 1970. The entire disclosure of this patent is incorporated herein by reference. Coates et. al. therein teach the method of preparation and the usefulness in anodic electrocoating compositions of a certain class of carboxyl-containing ethers of aminotriazine/aldehyde condensates derived from hydroxy alkyl carboxylic acids, in particular carboxyl-containing ethers of fully methylolated melamine and benzoguanamine and low polymers thereof. Their utility in (anodic) electrocoating compositions is ascribed to the fact that the carboxyl groups thereon will be ionizable in the aqueous electrocoating dispersion under the same conditions as the carboxyl groups on the binder resin and therefore will co-deposit more effectively with that resin upon an anode in contact with the dispersion. Since there would be numerous carboxylic acid groups on the binder resin electrodeposited on the anode, any enhancement of the acid catalysis of the curing reaction between the aminoplast resin and the binder resin by virtue of the small proportion of additional carboxylic acid groups present on the aminoplast resin would be expected to be hardly noticeable and was not mentioned by Coats et. al.
The use of aminoplast curing resin compositions in cathodic electrocoating wherein a coating is electrodeposited upon a cathode in contact with an aqueous dispersion of a cationic form of a binder resin stabilized by the presence of water soluble anion and also containing an aminoplast resin, is also known. Thus, Koral in U.S. Pat. No. 3,471,388 granted Oct. 7, 1969, the entire disclosure of which is incorporated herein by reference, discloses how to make and to use in both anodic and cathodic electrocoating processes melamine resins comprising a certain class of etherified methylolated melamine. The patent contains no suggestion of the existence or utility of acid-functional aminoplast resins, and in particular contains no suggestion of their utility in cathodic electrocoating.
A water soluble alkanolamine-terminated epoxy resin was disclosed by Wong et. al. in U.S. Pat. No. 3,336,253. It differs from the binder resins of the present invention in being expressly restricted to a single alkanolamine termined group and in not being substantially free of epoxide functionality and there is no suggestion of its use in electrocoating.